Here in the earliest text, we have two relative clauses where the verb form is the plural were. Joseph Smith emended the first of these to was in his editing for the 1837 edition; it appears that he interpreted the first relative pronoun which as referring to the immediately preceding “the record of Nephi”. But it is also possible that this which refers to the earlier “these things”, which is plural, thus supporting the use of were in the first relative clause (“these things … which were engraven on the plates”). In fact, there are five other passages in the text that clearly refer to “things” being engraven on plates:
It is even possible that the were here still refers to “the record of Nephi” since plural references to a singular record can be found elsewhere in the text (for some examples and discussion, see under 1 Nephi 5:21).
A couple of other emendations suggest themselves here in 3 Nephi 5:10: (1) the original text may have read which was for the first relative clause; but under the influence of the second which were, Oliver Cowdery accidentally wrote down which were in the first instance; (2) the original text may have read “the records of Nephi” but was accidentally changed to “the record of Nephi” under the influence of the preceding “my record of these things”. But since the earliest extant reading (the reading in 𝓟 and the 1830 reading) works here in 3 Nephi 5:10, the critical text will restore the original were in the first relative clause.
It should be noted here in 3 Nephi 5:10 that the record Mormon is making on his own plates is being taken from the large plates of Nephi (“the plates which were called the plates of Nephi”). This Nephi is the original Nephi, the son of Lehi, who made the plates (as described in 1 Nephi 9:1–4). On the other hand, Mormon’s account here in 3 Nephi 5:10 is based on “the record of Nephi” (that is, the record of the Nephi in 3 Nephi). In other words, the two Nephi’s mentioned in this passage are not the same person.
Summary: Restore the original were in the first relative clause in 3 Nephi 5:10: “I have made my record of these things according to the record of Nephi which were engraven on the plates which were called the plates of Nephi”; the phrase “according to the record of Nephi” intervenes between “these things” and the first relative clause (“which were engraven on the plates”), thus making it appear that the relative pronoun which refers to “the record of Nephi”.